1. Field of the Invention
The present invention relates generally to localized magnetic field detection and measurement and more particularly to magnetoresistive sensors.
2. Description of Related Art
Magnetic sensor technology is likely to play an enabling role in a wide variety of product applications. Advanced magnetic sensors will serve as read elements in magnetic heads for information retrieval from magnetic tape, disk and card storage media. The sensors may also serve as non-volatile information storage elements. In addition, magnetic sensors have great potential as switches and control elements in automobile engines and anti-lock braking systems as well as in medical applications to map magnetic signals from the human brain, and in underwater excavation and/or search systems to map the magnetic signature of metallic objects.
The value of magnetic sensor technology in future applications will depend in large part on the ability to develop sensors with improved sensitivity, smaller dimensions, and innovative packaging. Specifically, packaging refers to a system architecture which allows efficient signal retrieval without compromising performance or reliability.
Conventional magnetoresistive (MR) sensors use the magnetoresistance of simple thin films of ferromagnetic materials to detect and measure the local magnetic field in the environment of the sensor. Typically magnetic materials exhibit a change in electrical resistivity as the local magnetic field is changed. By passing an electrical current through the material, a change in resistivity produces a change in output voltage which is then a direct measure of the local magnetic field change. Such conventional MR sensors have been successfully used as information retrieval (i.e., read) sensor elements in magnetic heads used in magnetic disk storage devices. A magnetoresistance sensor may also be used as a reliable switching element for use as a device controller or a digital information storage element. The usefulness of conventional MR sensors is limited by the small magnitude of the MR response ##EQU1## where .DELTA.R is the change in resistance with and without an external saturation magnetic field and R is the resistance in the presence of the saturation magnetic field).
Recently materials with enhanced magnetoresistive response ##EQU2## have been produced by controlling the one-dimensional micro-structure of the materials. This effect, called giant magnetoresistance (GMR), is described in the IEEE Translation Journal on Magnetics in Japan, Vol.7, No. 9, September, 1992, pages 674-684 and IEEE Transactions on Magnetics, Vol. 28, No. 5, September, 1992, pages 2482-2487. The GMR effect has been produced in synthetic multilayered structures consisting of ferromagnetic metal layers separated by non-magnetic metal layers. The thicknesses of individual layers in these structures are typically several nanometers. The origin of the giant magnetoresistive effect is related to the large difference in resistivity between the configuration in which the magnetization state of adjacent ferromagnetic layers is substantially parallel versus the configuration in which adjacent ferromagnetic layers have a substantially anti-parallel magnetization state. The observed large variations in resistivity between these two states is thought to be due to the differences in spin dependent scattering at the layer boundaries when the magnetization states of adjacent ferromagnetic layers are parallel or anti-parallel.
While GMR materials provide the opportunity for sensors with a large magnetoresistive response ##EQU3## they typically do not have high sensitivity, ##EQU4## where: .DELTA.R=R(at H=O)-R(at H.sub.sat)
R=R(at H.sub.sat) PA1 H.sub.sat =the applied magnetic field to saturate the ferromagnetic layers.
This is because the anti-parallel magnetization state (which occurs at H=O, i.e., no applied external field) is produced by an exchange coupling between the adjacent ferromagnetic layers. The exchange coupling between adjacent ferromagnetic layers is a strong, short-range, quantum mechanical interaction. It requires a thin spacer layer between the ferromagnetic layers, and because it is so strong it demands a strong H.sub.sat in order to switch the sensor from the anti-parallel magnetization state to a parallel magnetization state. This is an important limitation in the practical application of GMR sensors to high density read/write magnetic heads because of the requirement to be sensitive to modest magnetic fields.